'const fooPtr' is the same as 'foo * const' (the pointer won't
change, but it's contents can).  But in general, if an interface
is trying to be const-correct, it should be using 'const foo *'
(the pointer is to data that can't be changed).

Fix up offenders in src/util outside of the virnet namespace.

Also, make a few virSocketAddr functions const-correct, for easier
conversions in future patches.

* src/util/virbuffer.h (virBufferError, virBufferUse)
(virBufferGetIndent): Use intended type.
* src/util/virmacaddr.h (virMacAddrCmp, virMacAddrCmpRaw)
(virMacAddrSet, virMcAddrFormat, virMacAddrIsUnicast)
(virMacAddrIsMulticast): Likewise.
* src/util/virebtables.h (ebtablesAddForwardAllowIn)
(ebtablesRemoveForwardAllowIn): Likewise.
* src/util/virsocketaddr.h (virSocketAddrSetIPv4Addr): Drop
incorrect const.
(virMacAddrGetRaw, virSocketAddrFormat, virSocketAddrFormatFull):
Make const-correct.
(virSocketAddrMask, virSocketAddrMaskByPrefix)
(virSocketAddrBroadcast, virSocketAddrBroadcastByPrefix)
(virSocketAddrGetNumNetmaskBits, virSocketAddrGetIpPrefix)
(virSocketAddrEqual, virSocketAddrIsPrivate)
(virSocketAddrIsWildcard): Use intended type.
* src/util/virbuffer.c (virBufferError, virBufferUse)
(virBufferGetIndent): Fix fallout.
* src/util/virmacaddr.c (virMacAddrCmp, virMacAddrCmpRaw)
(virMacAddrSet, virMcAddrFormat, virMacAddrIsUnicast)
(virMacAddrIsMulticast): Likewise.
* src/util/virebtables.c (ebtablesAddForwardAllowIn)
(ebtablesRemoveForwardAllowIn): Likewise.
* src/util/virsocketaddr.c (virSocketAddrMask, virMacAddrGetRaw)
(virSocketAddrMaskByPrefix, virSocketAddrBroadcast)
(virSocketAddrBroadcastByPrefix, virSocketAddrGetNumNetmaskBits)
(virSocketAddrGetIpPrefix, virSocketAddrEqual)
(virSocketAddrIsPrivate, virSocketAddrIsWildcard)
(virSocketAddrGetIPv4Addr, virSocketAddrGetIPv6Addr)
(virSocketAddrFormat, virSocketAddrFormatFull): Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>